NATURAL GAS CONVERSIONV Studies in Surface Science and Catalysis,Vol. 119 A. Parmalianaet al. (Editors) 1998 Elsevier Science B.V.
Oxidative dehydrogenation o f propane on supported redox and acid-base properties
The role o f
F. Arena a, F. Frusteri b, A. Parmaliana a, G. Martra c and S. Coluccia c a. Dipartimento di Chimica Industriale, Universit/t degli Studi di Messina, Salita Sperone c.p. 29, 1-98166 S. Agata (Messina), Italy b. Istituto CNR-TAE, Salita S. Lucia 39, 1-98126 S. Lucia (Messina), Italy c. Dipartimento di Chimica Inorganica, Chimica Fisica e Chimica dei Materiali, Universitb. degli Studi di Torino, via P. Giuria 7, 1-10125 Torino, Italy
The influence of several oxide carriers on the activity of vanadia catalysts in the oxidative dehydrogenation of propane (POD) has been investigated. The effects of both redox and acidbase properties of the support on the surface structures and reactivity of vanadia have been assessed. The occurrence of competitive reaction paths on bare oxide carriers and supported vanadia catalysts has been discussed.
1. INTRODUCTION The production of olefins by the oxidative dehydrogenation of light alkanes represents a very attractive alternative to conventional dehydrogenation processes since it could allow the use of natural gas as feedstock for production of large scale chemicals actually obtained from oil. Intensive research efforts recently focused on such topic have documented that supported vanadia systems are active and selective catalysts for attaining the oxidative dehydrogenation of light paraffins. The performance of such V205 systems is strongly affected by the nature of the oxide carrier and V205 loading/1-3/. Although the coordination of VV-species mostly determines the redox properties and then the reactivity of the active phase /1/, also the influence of the acid-base features of supported vanadia catalysts on the oxidative dehydrogenation reactions has been claimed/1,2,4/. Then, here we report on the effects of several oxide carriers (i.e., MgO, A1203, TiO2, ZrO2, SiO2 and HY zeolite) characterised by different surface acid-base properties on the performance of same loaded (4.7-5.3 wt%) V205 catalysts in the POD reaction. 2. EXPERIMENTAL 2.1 Catalysts were prepared by incipient wetness of commercial oxide (i.e., MgO, A1203, TiO2, ZrO2, SiO2 and HY zeolite) carriers with an aqueous solution (pH=l 1) of ammonium
666 metavanadate. All samples were dried at 110~ and then calcined in air at 600~ The list of samples along with their notation and physico-chemical properties is reported in Table 1. 2.2 Catalytic measurements in the POD were carried out in a flow quartz microreactor connected on line with a GC equipped with a three column analytical system/3/. The activity of the catalysts has been evaluated under the following conditions: TR, 500~ PR, 1 bar; Wcat, 0.01-0.25 g (diluted with SiC (1/10, w/w)). The molar composition of the reaction mixture was C3Hs:O2:N2:He=2:I:I:8 and a total flow rate of 100 STP cm3.minq was used. 2.3. Catalyst characterization 2.3.1. TPR measurements in the range 200-1000~ have been performed in a flow apparatus operating in both continuous and pulse modes using a 6% H2/Ar mixture flowing at 60 STP cm3/min and a heating rate of 20~ 2.3.2. High Temperature Oxygen Chemisorption (HTOC) tests/5,6/were carried out at 367~ in a pulse mode after reduction of catalysts for 2h in H2 at the same T. Vanadia dispersion (O/V) was calculated assuming a chemisorption stoichiometry of O2:V=1:2.
Table 1. List of samples Supports Code Composition
(m2.~"1) M A T Z S H
A1203 TiO2 ZrO2 SiO2 HY zeolite
24 185 51 37 381 632
VM-5 VA-5 VT-5 VZ-5 VS-5 VH-5
Catalysts V205 loading S.A.BET
4.7 4.8 50 53 50 53
101 209 41 34 286 350
Theor. surf. cov.
(Vat..nm"2) 3.08 1.52 8.07 10.32 1.16 1.00
3. RESULTS and DISCUSSION 3.1. Catalytic pattern of oxide supports and supported vanadia catalysts. The catalytic behaviour pattern in POD of the various oxide supports and vanadia catalysts has been evaluated operating with different contact times (x) to attain a differential C3H8 conversion. The activity data of the various systems are summarised in Table 2 in terms of C3Hs conversion, reaction rate values, C3H6 and CO• selectivity and C3H6 productivity (STY, gc3H6"kgcat'h'l). C3H8 conversion never exceeds 6% while 02 conversion is quite larger even if it keeps always less than 40%. All the catalysts produce essentially C3I-I6, COx and minor amounts of C2 hydrocarbons (1-3%) and oxyproducts (1-7%). Bare oxide supports exhibit rather different catalytic properties in POD, indeed differences in specific activity (SA, 0.4-10.1 ~molc3Hs-g~.s~), specific surface activity (SSA, 1.1-197 nmolc3Hs's~'m 2) and propylene selectivity (0-55%) up to ca. two orders of magnitude have been observed. In particular, titania exhibits the best functionality in POD with the highest values of SA (10.1 l.tmolC3HS"gl's1) and 5C3H6(55%) which account for a STY equal to 833 gc3H6"kgcat'hl. The HY zeolite bears SA (8.9 lamolc3Hs.gl.sl) and SC3H6(44%) values close to
667 those of titania and correspondingly a comparable STY (599 gc3H6.kgcat.h'l). However, this system shows a fast deactivation likely due to a strong adsorption of reagents/products resulting in poisoning/coking phenomena. Zirconia and alumina show a comparable SA (2.43.5 [.tmolc3ns"g'l's"l) being less reactive than previous systems; the Sc3H6 is 19 and 35% respectively accounting for much lower STY's (74-180 gc3u6-kgcat'h'l). Magnesia and silica are the least active systems with SA values of 0.6 and 0.4 ].tmolc3Hs.gl.s-1, respectively. Silica displays a 5C3H6 (38%) comparable with that of the other oxides, while no propylene formation is detected on magnesia. However, taking into account that the above oxides are characterized by rather different surface area values, a more reliable comparison of their catalytic activity, made on the basis of the SSA, provides the following reactivity scale: T >>Z >>M > A > H >>S. Table 2. Oxidative dehydrogenation of propane on V2Os-based catalysts R e a c t i o n rate Sample C3H8 conv. SC3H6 Scox (nmolc3Hs"m2"s~) (%) (Ittmolc3Hs'g'l'S"l) M 1.2 0.6 27.1 100 VM-5 4.3 46.4 31 68 4.7 18.2 35 62 A 6.2 3.4 104 73 26 VA-5 3.2 21.8 197 55 48 T 3.7 10.1 1,690 33 67 VT-5 5.1 69.5 69.2 19 79 Z 4.7 2.5 VZ-5 2.5 34 1 1,000 53 46 S 0.8 0.4 1.1 38 32 12.6 72 19 VS-5 3.3 36 H 3.3 14.2 44 55 89 VH-5 4.4 34.2 38 60 11.9
STY (gc3H6-kgcat'h"l) 0 222 180 2407 833 3200 74 2356 25 394 599 690
Vanadia addition strongly promotes the functionality of all the oxide carriers giving rise to an enhancement of specific activity and STYc3H6. The VT-5, paralleling the reactivity of titania support, is the most active system with a SA equal to 69.5 ktmolc3Hs'gq's q which coupled with a Sc3H6 of 33% leads on the whole to the highest STY (3200 gc3H6"kgcat'h'l). VZ-5 sample shows a SA lower by ca. one half than that of the VT-5 catalyst and a higher selectivity to propylene (53%), much larger also than that of the relative carrier, accounting for a STY value of ca. 2400 gc3H6"kgcat'hq. VA-5 exhibits a SA still lower than that of previous systems even if the STY keeps equal to that of the VZ-5 sample mainly as a consequence of the highest 5C3H6 (73%). All the other systems (VH-5, VS-5 and VM-5) are characterised by SA values lower than that of VA-5 catalyst. As the selectivity to propylene ranges from 72 (VS-5) to 31% (VM-%), on the whole such systems ensure much lower STY's (222-690 gc3H6"kgcat'h'l). Analogously to the relative support, a fast deactivation of the VH-5 catalyst has been observed. Furthermore, taking into account the SSA of the various systems, the following reactivity scale is obtained: VT-5 >VZ-5 >> VA-5 > VM-5 > VH-5 >VS-5.
668 However, the fact that the strongly promoting effect induced by vanadia on the activity of supports is accompanied by a concomitant enhancement in the 5C3H6 proves that a more effective reaction mechanism occurs on supported vanadia systems.
3.2. Reducibility, dispersion and surface structures of supported vanadia catalysts. The redox properties and dispersion of supported vanadia catalysts have been comparatively probed by TPR and HTOC to achieve basic insights into their surface structures/5,6/. The TPR pattern of vanadia catalysts and relative oxide support are shown in Figure 1. All such spectra monitor a complex pattern consisting of several peaks associated with the reduction of different surface species whose reducibility is controlled by the surface physico-chemical properties of the relative oxide carriers/1,2,5/. Namely, the promotional effect of the oxide carrier on the reduction of vanadia depends upon its surface reducibility as it determines the reactivity of the bridging "V-O-support" fimctionalities /1,5/. TiO2 shows a reduction profile accounting for an enhanced reducibility both at the surface and in the bulk, whilst the fiat TPR profiles of HY zeolite, SiO2, ZrO2, A1203, and MgO indicate that o such systems are essentially not reducible. ~. i i ~ i ~ ~VA-5 However, further insights into the surface reduction characteristics of metal oxide carriers can be achieved from the TPR of supported vanadia P,I systems/5/. In fact, the reducibility scale of V205 ~o -c catalysts: VT-5 > VZ-5 > VA-5 > VS-5 > VH-5 > VM-5, the same either based on the onset T of reduction (i.e., To,red) or T of the first peak maximum/5/, is in a very good agreement with literature data /1/, i i : pointing to the reactivity of the surface oxygen layer of the support as factor governing the reducibility of vanadia catalysts. Then, while the 300 400 500 600 700 800 900 above reducibility scale should account for an Temperature (~ enhanced redox behaviour of TiO2 and ZrO2 Figure 1. TPR pattern of supported vanadia explaining their highest SSA, it must be stressed catalysts ( - - ) and relative support (--). that some other factor(s) control(s) the surface reactivity of other bare oxide carriers in POD. Although surface redox properties of oxide carriers allow to explain the different reduction features of V205 catalysts, the above reducibility scale can be also rationalised in terms of acid-base properties of the carrier. Indeed, as shown in Figure 2, plotting the values of To,redof the catalysts vs. the pH of zero point charge (i.e., pHzpc) of the relative oxide carrier/5/a hollow trend with the minimum for a pHzpcclose to 6 is found. Taking into account the easiest reduction of tetrahedrically coordinated (Td) "isolated" species and that the support does not significantly affect their molecular structure and coordination symmetry/1,2,5/, these findings indicate that oxide carriers bearing either a strong surface acidity or basicity depress the reduction of the isolated vanadyl species. In particular, it can be inferred that the surface
669 acidity of silica and HY zeolite exerts an electron-acceptor effect which hinders the reduction of monolayer species/5/. Whereas, a strong interaction between basic MgO 425 and acidic V205, resulting in the extensive formation of vanadates/1,2/, renders difficult the reduction of the VM-5 system. VM-5 T / On amphoteric oxides like TiO2, ZrO2 and A1203, VH-5 i the electron-donor positive effect of weakly basic , / 375 sites should be prevalent enabling the prompt \~I' VS-5 e reduction of VO43" units, while the intrinsic 0 , / o reducibility of TiO2 further promotes the -o" \ / reduction of the VT-5 catalyst to lower T. t Therefore, even if the structure of the monolayer \ , ! species in the calcined catalysts is not affected by 325 the nature of the support/1/, it appears that the \, vA-s ,,/' surface acid-base features of the oxide carrier influence the electronic properties and reduction VZ-5 ~ - J " pattern of mono-oligomeric species besides to 0 V'I'-5 their maximum development /1,2/. Indeed, the 275 HTOC data, shown in Table 3, reveal that 2 4 6 8 10 vanadia dispersion on various catalysts ranges between 0.14 and 0.63. VA-5 catalyst is PS74~c Oxide Support characterized by the highest vanadia dispersion Figure 2. Relationship between the onset T of (0.63), while similar oxygen uptakes account for reduction (To,red) of vanadia catalysts and the point of zero charge (PH~c) of the support. analogous dispersion values on VZ-5 (0.46) and VS-5 (0.49) samples. A dispersion value of 0.38 is found for the VT-5 system, even if this is an underestimated value/5/likely as a consequence of the fact that the formation of a V-Ti-O solid solution/1/ renders VO o r 4)+ stable against oxidation. The VH-5 and VM-5 samples exhibit the lowest oxygen uptakes accounting for dispersion values equal to 0.14 and 0.17 respectively. e
Table 3. HTOC of supported V205 catalysts: Sample 02 uptake O/V
2 uptake and vanadia dispersion (O/V) Sample 02 uptake O/V
) VM-5 VA-5 VT-5
43.3 165.4 106.6
) 0.17 0.63 0.38
VZ-5 VS-5 VH-5
133.8 134.9 40.5
0.46 0.49 0.14
As it has been previously ascertained that irrespective of the nature of the support /2,6/, oxygen chemisorption occurs on V 3+ sites coming from the reduction of Td mono-oligomeric species/5,6/, it is evident that the acid-base properties of the oxide support play a crucial role on the reducibility of vanadia because they control the development of the monolayer and thus the dispersion of the active phase which depends upon both vanadia-support interaction and hydroxyl population of the carrier/1,5/.
3.3. Redox and acid-base properties and catalytic pattern of oxide supports and supported vanadia catalysts. On the basis of catalytic data it is evident that no relationship between acid-base character of the oxide carrier and SA and SSA o r Sc3H6 exists. Moreover, vanadia acts as a promoter of both activity and selectivity pointing to its capability to enable a surface concerted mechanism involving the activation of propane molecule and gas-phase oxygen mostly over Td mono-oligomeric sites /1,3/. This indicates the prevailing redox behaviour of vanadia catalysts as documented by Figure 3, showing the inverse relationships linking both SA and STY (A) with To,redas well as SSA and Surface yield (B) with To,red. .
,~ 6 0 ~ i -'.t 4 | 9 ............. ~:..................... ..: .................... .: . - 3 0 0 0
................ ~ ,~.i................. _~,,,,~ ....................... i.. ~ 1000 :
nli . . . .
.................... 415o ~
'"~'~'-'-~-~ In 360-
T o ,red. (0 C)
3. A : R e l a t i o n s h i p s
....... ' , ~ ...... ~..................... ~.................... L 3 0 0 !"
-~ 1.0 m
", ,' [u r v ........ ,; ........ -'~,. . . . ._. . . . . .-............................ ..... 2000
oo 2 0
. . . . . . . . . . . .
To,re d. B" R e l a t i o n s h i p s
Such a mechanism is also prevalent on TiO2 and could less effectively occur on ZrO2. By contrast, on bare A1203, MgO and HY zeolite, possessing either basic or acidic Br6nsted sites, a surface-assisted reaction mechanism, involving the primary stabilization of adsorbed carbocation and carbanion on acidic and basic sites respectively, should prevail. In particular, it is inferred that carbocation would react leading to the formation of C3H6, whereas the further reaction of the carbanion, formed on the basic sites of MgO, would mainly give the formation of carbon oxides. On silica /5/ and magnesia, however, a poor population of Br0nsted sites should account for their very low surface reactivity in the POD reaction.
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